The final vote count showed Cassidy with 56 percent and Landrieu with 44 percent of the vote.

"This victory belongs to you," Cassidy told his supporters in Baton Rouge. "The people of Louisiana voted for a government that serves us but does not tell us what to do."

Cassidy's win completes the GOP's conquest of the Senate. Republicans will have 54 seats in the chamber starting in January, and Democrats will control 46 seats. The runoff result gives the GOP its ninth Senate-seat pickup in this year's elections.

Landrieu thanked her supporters at her election headquarters in New Orleans.

"The joy has been in the fight," she said, recalling her battles to send federal money to Louisiana after the 2005 hurricanes and the 2010 BP oil spill. "It's been a blessing. It's been a fight worth waging... I could never ever have started this race, or finished it, if it wasn't for a solid and secure faith in God Himself, and without my family."

Landrieu, the only Democratic senator from the Deep South and the only remaining Democrat elected statewide in Louisiana, worked during her campaign to distance herself from Obama, noting that she had opposed the administration's moratorium on deep-water drilling in the Gulf of Mexico after the 2010 BP oil spill, and supported construction of the Keystone XL pipeline.

Cassidy, meanwhile, repeatedly told voters Landrieu has voted in line with Obama's positions 97 percent of the time, and reminded them that she voted in 2010 for the Affordable Care Act, which is unpopular in Louisiana.

That strategy seemed to be effective. An average of polling data compiled by RealClearPolitics had Cassidy leading by 20 points heading into the runoff.

"Unless (Cassidy) was convicted of treason, he was going to win," Louisiana pollster Bernie Pinsonat said.

Cassidy's obvious edge helped convince national Democratic Party committees that Landrieu wasn't a solid investment and their money would be better used elsewhere. Landrieu was badly outspent in the weeks leading up to the runoff.

Landrieu, 59, focused on maximizing turnout among black voters, but the breakdown of people casting early ballots suggested they would not participate in large numbers.

Landrieu's defeat after three terms in the Senate marks a turning point in her family's dynastic influence in Louisiana. Her father, Moon Landrieu, is a former New Orleans mayor, and her brother, Mitch Landrieu, is the city's current mayor.

The Senate race was expected to be the most expensive in the state's history, political experts said. By mid-October, spending had topped $24.5 million, according to the nonpartisan Center for Responsive Politics. The $13.2 million spent by the candidates at that point compared to $16 million in total candidate expenditures in the 2008 Senate race in Louisiana, and $15.9 million in 2002.

No candidate in the Senate race took more than 50 percent of the vote on Nov. 4, setting up Saturday's runoff. Landrieu took 42.1 percent and Cassidy took 41 percent. A second Republican on the ballot, Rob Maness, took 13.8 percent.

The race was marked by waves of negative ads by each side. Landrieu accused Cassidy, 57, a part-time teaching physician at LSU, of charging the school for hours that he never worked (Cassidy denied any wrongdoing). Cassidy and his supporters attacked Landrieu for improperly spending taxpayer money on charter flights that included campaign stops (Landrieu apologized and said her campaign reimbursed the federal government for the flights).

With Landrieu's hopes for re-election appearing in serious peril, Democratic leaders in the Senate arranged to hold a vote on her legislation to approve the Keystone pipeline, hoping passage of the bill would boost her chances in the runoff. But the bill came one vote short of passing, a stinging defeat. A similar bill backed by Cassidy, meanwhile, passed in the House.

Landrieu's campaign touted her chairmanship of the Senate Energy and Natural Resources Committee, but once it was clear the Nov, 4 elections had given Republicans control of the Senate in the next Congress, her committee post essentially became a non-issue.

Landrieu, the only Democratic senator from the Deep South and the last remaining Democrat elected statewide in Louisiana, worked during her campaign to distance herself from Obama, noting that she had opposed the administration's moratorium on deep-water drilling in the Gulf of Mexico after the 2010 BP oil spill, and supported construction of the Keystone XL pipeline.

Cassidy, meanwhile, repeatedly told voters Landrieu has voted in line with Obama's positions "97 percent of the time," and reminded them that she voted in 2010 for the Affordable Care Act, which is unpopular in Louisiana.

That strategy seemed to be effective. An average of polling data compiled by RealClearPolitics had Cassidy leading by 20 points heading into the runoff.

Jeffrey Sadow, a political scientist at Louisiana State University at Shreveport, said Landrieu's vote for the 2010 health care law "basically kind of broke the dam" in terms of her ability to win election to a fourth term.

Sadow said both candidates did a good job with the hands they were dealt.

"It's just that Cassidy had a much better hand to begin with, and he didn't squander it,.'' he said.

Cassidy's obvious edge in the race helped convince national Democratic Party committees that Landrieu wasn't a solid investment and their money would be better used elsewhere. Landrieu was badly outspent in the weeks leading up to the runoff.

Landrieu, 59, focused on maximizing turnout among black voters, but the breakdown of people casting early ballots suggested they would not participate in large numbers.

Landrieu's defeat after three terns in the Senate would mark a turning point in her family's dynastic influence in Louisiana. Her father, Moon Landrieu, is a former New Orleans mayor, and her brother, Mitch Landrieu, is the city's current mayor.

The Senate race was expected to be the most expensive in the state's history, political experts said. By mid-October, spending had topped $24.5 million, according to the nonpartisan Center for Responsive Politics. The $13.2 million spent by the candidates at that point compared to $16 million in total candidate expenditures in the 2008 Senate race in Louisiana, and $15.9 million in 2002.

No candidate in the Senate race took more than 50 percent of the vote on Nov. 4, setting up Saturday's runoff. Landrieu took 42.1 percent and Cassidy took 41 percent. A second Republican on the ballot, Rob Maness, took 13.8 percent.

The race was marked by waves of negative ads by each side.

With Landrieu's hopes for re-election appearing in serious peril, Democratic leaders in the Senate arranged to hold a vote on her legislation to approve the Keystone pipeline, hoping passage of the bill would boost her chances in the runoff. But the bill came one vote short of passing, a stinging defeat. A similar bill backed by Cassidy, meanwhile, passed in the House.

Landrieu's campaign touted her chairmanship of the Senate Energy and Natural Resources Committee, but once it was clear the Nov. 4 elections had given Republicans control of the Senate in the next Congress, her committee post essentially became a non-issue.